The capitulation was ungracious, the invitation grudging. Cordelia could guess what it had cost her to give it. She said impulsively:

'I could come on Sunday instead. I've got a car, I could get here sooner.'

It would be a day lost to Sir Ronald Callender, but she wouldn't charge him. And even a private eye was surely entitled to a day off on Sundays.

'He won't want a slip of a girl. There's things to do for him that need a man. He took to that boy. I could see that. Tell him he can come.'

Cordelia turned to her.

'He would have come, I know he would. But he can't. He's dead.'

Mrs Gladwin did not speak. Cordelia put out a tentative hand and touched her sleeve. There was no response. She whispered:

'I'm sorry. I'll go now.' She nearly added 'If there's nothing I can do for you,' but stopped herself in time. There was nothing she or anyone could do.

She looked back once as the road bent towards Bury and saw the rigid figure still at the gate.

Cordelia wasn't sure what made her decide to stop at Bury and walk for ten minutes in the Abbey gardens. But she felt she couldn't face the drive back to Cambridge without calming her spirits and the glimpse of grass and flowers through the great Norman doorway was irresistible. She parked the Mini on Angel Hill, then walked through the gardens to the river bank. There she sat for five minutes in the sun. She remembered that there was money spent on petrol to be recorded in her notebook and felt for it in her bag. Her hand brought out the white prayer-book. She sat quietly thinking. Suppose she had been Mrs Callender and had wanted to leave a message, a message which Mark would find and other searchers might miss. Where would she place it? The answer now seemed childishly simple. Surely somewhere on the page with the collect, gospel and epistle for St Mark's Day. He had been born on April 25th. He had been named after the Saint. Quickly she found the place. In the bright sunlight reflected from the water she saw what a quick rustle through the pages had missed. There against Cranmer's gentle petition for grace to withstand the blasts of false doctrine was a small pattern of hieroglyphics so faint that the mark on the paper was little more than a smudge. She saw that it was a group of letters and figures.

EMC

AA

14.1.52

The first three letters, of course, were his mother's initials. The date must be that on which she wrote the message. Hadn't Mrs Goddard said that Mrs Callender had died when her son was about nine months old? But the double A? Cordelia's mind chased after motoring associations before she remembered the card in Mark's wallet. Surely these two letters under an initial could only show one thing, the blood group. Mark had been B. His mother was AA. There was only one reason why she should have wanted him to have that information. The next step was to discover Sir Ronald Callender's group.

She almost cried out with triumph as she ran through the gardens and turned the Mini again towards Cambridge. She hadn't thought out the implications of this discovery, or even whether her arguments were valid. But at least she had something to do, at least she had a lead. She drove fast, desperate to get to the city before the post office closed. There, she seemed to remember, it was possible to get a copy of the Executive Council's list of local doctors. It was handed over. And now for a telephone. She knew only one house in Cambridge where there was a chance of being left in peace to telephone for up to an hour. She drove to 57, Norwich Street.

Sophie and Davie were at home playing chess in the sitting-room, fair head and dark almost touching over the board. They showed no surprise at Cordelia's plea to use the telephone for a series of calls.

'I'll pay, of course. I'll make a note of how many.'

'You'll want the room to yourself, I expect?' said Sophie. 'We'll finish the game in the garden, Davie.'

Blessedly incurious they carried the chess board with care through the kitchen and set it up on the garden table. Cordelia drew a chair to the table and settled down with her list. It was formidably long. There was no clue where to begin but perhaps those doctors with group practices and addresses near the centre of the city would be the best bet. She would start with them, ticking off their names after each call. She remembered another reported pearl of the Superintendent's wisdom: 'Detection requires a patient persistence which amounts to obstinacy.' She thought of him as she dialled the first number. What an intolerably demanding and irritating boss he must have been! But he was almost certainly old now – forty-five at least. He had probably eased up a bit by now.

But an hour's obstinacy was unfruitful. Her calls were invariably answered; one advantage of ringing a doctor's surgery was that the telephone was at least manned. But the replies, given politely, curtly or in tones of harassed haste by a variety of respondents from the doctors themselves to obliging daily women prepared to convey a message, were the same. Sir Ronald Callender was not a patient of this practice. Cordelia repeated her formula. 'I'm so sorry to have troubled you. I must have misheard the name.'

But after nearly seventy minutes of patient dialling she struck lucky. The doctor's wife answered.

'I'm afraid you've got the wrong practice. Dr Venables looks after Sir Ronald Callender's household.'

This was luck indeed! Dr Venables wasn't on her preliminary list and she wouldn't have reached the V's for at least another hour. She ran her finger down the names and dialled for the last time.

It was Dr Venables' nurse who answered. Cordelia spoke her prepared piece:

'I'm ringing for Miss Learning from Garforth House. I'm sorry to trouble you but could you please remind us of Sir Ronald Callender's blood group? He wants to know it before the Helsinki Conference next month.'

'Just a minute, please.' There was a brief wait; the sound of footsteps returning.

'Sir Ronald is Group A. I should make a careful note of it if I were you. His son had to ring a month or so ago with the same inquiry.'

'Thank you! Thank you! I'll be careful to make a note.' Cordelia decided to take a risk.

'I'm new here, assisting Miss Learning, and she did tell me to note it down last time but stupidly I forgot. If she should happen to call, please don't tell her that I had to trouble you again.'

The voice laughed, indulgent to the inefficiency of the young. After all, it wasn't likely to inconvenience her much.

'Don't worry, I shan't tell her. I'm glad she's got herself some help at last. Everyone's well, I hope?'

'Oh, yes! Everyone's fine.'

Cordelia put down the receiver. She looked out of the window and saw that Sophie and Davie were just finishing their game and were putting the pieces back in the box. She had just finished in time. She knew the answer to her query but she still had to verify it. The information was too important to leave to her own vague recollection of the Mendelian rules of inheritance gleaned from the chapter on blood and identity in Bernie's book on forensic medicine. Davie would know, of course. The quickest way would be to ask him now. But she couldn't ask Davie. It would mean going back to the public library, and she would have to hurry if she were to be there before it closed.

But she got there just in time. The librarian, who by now had got used to seeing her, was as helpful as ever. The necessary reference book was quickly produced. Cordelia verified what she had already known. A man and wife both of whose bloods were A could not produce a B group child.

Cordelia was very tired by the time she got back to the cottage. So much had happened during one day; so much had been discovered. It seemed impossible that less than twelve hours previously she had started out on her search for Nanny Pilbeam with only a vague hope that the woman, if she could be found, might provide a clue to Mark Calender's personality, might tell her something about his formative years. She was exhilarated by the success of the day, restless with excitement, but too mentally exhausted to tease out the tangle of conjecture which lay knotted at the back of her mind. At present the facts were disordered; there was no clear pattern, no theory which would at once explain the mystery of Mark's birth, Isabelle's terror, Hugo and Sophie's secret knowledge, Miss Markland's obsessive interest in the cottage, Sergeant Maskell's almost reluctant suspicions, the oddities and unexplained inconsistencies which surrounded Mark's death.